This application relates generally to the field of power generation, and more particularly to the field of combined cycle power plants.
Combined cycle power plants are well known in the art. A combined cycle power plant includes both a gas turbine-based topping cycle and a steam turbine or a steam rankine bottoming cycle that is driven by heat in the exhaust of the gas turbine engine. U.S. Pat. No. 6,145,295, incorporated by reference herein, describes one such combined cycle power plant.
During startup of a combined cycle power plant from cold start conditions, the gas turbine portion of the plant necessarily must be started before the steam turbine portion. The term cold start is a relative term but is used herein to refer generally to conditions where the plant has not been operated for an extended time period, such as 48 hours, and where the boiler in not pressurized. During startup of an industrial gas turbine having a single shaft-constant speed arrangement, there is a relatively rapid increase in the flow rate of the exhaust from the gas turbine as it accelerates to operating speed. Thereafter, the exhaust gas flow rate remains relatively constant except for the effect of compressor inlet guide vane modulation. After the gas turbine reaches operating speed, the temperature of the exhaust gas gradually increases as the firing temperature of the gas turbine is increased up to the level required to produce the desired power output. Modern gas turbine engines may have firing temperatures in excess of 1,400xc2x0 C. at full rated power. However, the rate of increase in load and temperature of the gas turbine exhaust is constrained by thermal transient stress limits in the components of the steam turbine and the balance of plant, including the heat recovery steam generator (HRSG) that is exposed to the hot exhaust gas stream. Note that the terms heat recovery boiler, heat recovery steam generator, HRSG, and boiler are used herein to include both drum and drumless boilers (once through steam generators).
Although the gas turbine exhaust flows through the HRSG during the startup of the gas turbine, a considerable period of time elapses before an initially cold HRSG is capable of generating steam that is sufficient to roll the steam turbine and having adequate pressure and superheat for expansion in the turbine without unacceptable moisture generation. The startup temperature of the gas turbine exhaust is regulated to gradually heat and pressurize the HRSG. In a typical combined cycle plant, the gas turbine may be initially limited to about 20-30% rated power in order to maintain the exhaust at a sufficiently low temperature to maintain stresses within acceptable levels in the cold HRSG. During this heating period, the steam turbine and associated auxiliary systems are also heated gradually by bleeding a small quantity of the steam produced by the HRSG through the steam turbine and into the condenser. However, the quantity of steam produced by the HRSG at such a low gas turbine power level is insufficient to initiate steam turbine roll and to accelerate the steam turbine to operating speed. Thus, the startup of the steam turbine is delayed until both the HRSG and the steam turbine are brought up to temperature with the gas turbine operating at limited power levels. As the system temperature increases, the gas turbine power level is gradually increased until eventually the quantity and quality of steam produced by the HRSG is sufficient to support the initial roll of the steam turbine.
The necessity to gradually heat a combined cycle power plant during startup reduces the overall efficiency of the plant and reduces the plant""s ability to respond to rapidly changing power requirements. Furthermore, the operation of the gas turbine portion of the plant at less than full rated load may result in a level of gaseous emissions that exceeds regulatory or Original Equipment Manufacturers base load contractual requirements. In particular, it is known that the level of carbon monoxide produced in a gas turbine engine will increase as the firing temperature is decreased during part-load operation. Operation of the gas turbine portion of a combined cycle power plant at 20-50% rated load during the startup phase will often place the plant outside of emissions compliance limits. Not only does such operation have an undesirable impact on the local environment, but it may also have a negative financial impact on the owner or operator of the plant, since a plant revenue stream may be adversely impacted by operation outside of regulatory compliance limits. Certain power pool arrangements may provide that the owner/operator of the plant does not receive any payment for the power produced during periods of noncompliance with emissions regulations. Accordingly, there is a strong incentive to reduce the startup time for a combined cycle power plant and/or to reduce the operation of the plant at non-compliance emissions points.
U.S. Pat. No. 5,412,936, incorporated by reference herein, describes a method of effecting startup of a cold steam turbine system in a combined cycle power plant. This method requires steam to be extracted from an intermediate location in the superheater portion of the HRSG. In one embodiment, this system utilizes a gas turbine exhaust bypass damper to direct a portion of the gas turbine exhaust away from the HRSG in order to allow the gas turbine to be operated at any load during the startup of the steam turbine.
U.S. Pat. No. 5,473,898, also incorporated by reference herein, describes a method and apparatus for warming a steam turbine in a combined cycle power plant. Compressed air from the compressor section of the gas turbine is used to warm the steam turbine from cold conditions. This allows the turbine warming process to begin at an earlier time than would otherwise be possible when using only steam produced by the HRSG as the heat source.
Further improvements are desired to reduce the time necessary for starting a combined cycle power plant, to increase the amount of power produced and to ensure compliance with emissions requirements during the startup process.
Accordingly, a combined cycle power generating apparatus is described herein as including: a gas turbine portion producing an exhaust gas; a heat recovery apparatus receiving the exhaust gas and producing steam; a steam turbine receiving the steam; and an air injection apparatus combining a flow of injection air with the exhaust gas upstream of the heat recovery apparatus. The air injection apparatus may be an air ejector having a motive air inlet in fluid communication with a compressor of the gas turbine portion for receiving compressed air, and having an ambient suction air inlet for receiving ambient air for combining with the compressed air to produce the injection air. Alternatively, the air injection apparatus may be a fan.
A method of starting a combined cycle power plant is described herein. The plant includes (i) a gas turbine portion comprising a compressor for producing compressed air and a turbine for producing exhaust gas, (ii) a heat recovery steam generator for producing steam by transferring heat from the exhaust gas to a flow of feed water, and (iii) a steam turbine for expanding the steam, and the method includes: starting the gas turbine portion to produce a flow of exhaust gas; producing a flow of injection air; and combining the flow of injection air with the flow of exhaust gas to produce an augmented exhaust stream directed through the heat recovery steam generator to produce a flow of steam for starting the steam turbine. The method may further include: producing a flow of compressed air in the compressor; and using a portion of the flow of compressed air produced in the compressor to produce the flow of injection air.